FIG. 1 illustrates an aircraft engine 3 of the unducted fan type, in which the invention can be used. Region 6 is shown in FIG. 2, wherein counter rotating turbines 9 (hatched) and 12 (plain) are driven by a hot gas stream 13 provided by a core engine (not shown). The turbines 9 and 12 act as sources of motive power and drive counter rotating fan blades 15 and 18 shown in FIGS. 1 and 2. (The term "counter rotating" means that turbines 9 and 12, as well as blades 15 and 18 which are attached to them, rotate in opposite directions, and also that the turbines and blades are coaxial.)
A view of subregion 21 in FIG. 2 is shown in FIG. 3, and in more detailed form in FIG. 4. The turbine blades which are located in subregion 6B in FIG. 2 are shown schematically in FIG. 3 as blades 28 and shown in detail in FIG. 4. The blades 28 extend between a casing 24 and an inner barrel 92 in FIG. 4. The blades extract energy from the air stream 13 in FIG. 2 and also act as a stiff connecting web between the casing 24 and the barrel 92.
The fan blades 15 in FIG. 3 are supported by a structure which is shown as a ring 22 in FIG. 4, and which is fastened to the casing 24 by brackets 25. During operation, the centrifugal load of the fan blades 15 is carried by the ring 22 as a hoop stress.
The actual structure used is not the circular ring 22 shown in FIG. 4, but is what is termed a polygonal ring 22P shown in FIG. 3A. The polygonal ring 22P includes two types of sections: one type is a blade support section 22B, or "hub." A schematic hub 22B is shown in FIGS. 2 and 5, and includes thrust bearings 109 which transmit the centrifugal load imposed by the fan blade 15 to the ring 22P. The thrust bearings 109 allow pitch change of the blade to occur, as indicated by arrow 23.
The other type of ring section is a connector 22A in FIG. 3A which connects neighboring hubs 22B. The connector 22A includes rails 29 which are in tension because of the centrifugal load of the blades 18.
One type of polygonal ring is described in the U.S. Pat. No. 4,863,352 entitled "Blade Carrying Means," filed by Hauser, Strock, Morris, and Wakeman on Nov. 2, 1984, and having Ser. No. 667,663. This application is hereby incorporated by reference.
In order to cause the pitch change indicated by arrow 23 in FIGS. 2 and 5, a large torque must be applied to the blades 15 and 18. The mechanical source of the torque is located within the turbines 9 or 12 in FIG. 2, such as at a region 104. The torque is transmitted from region 104 to the blade 15 by a shaft 101.
If the large torque required were delivered through shaft 101, the shaft must have a large diameter for structural integrity, system stiffness, and proper response. This large shaft must be concealed within a turbine blade 28 in FIG. 4. Such concealment requires configuring the turbine blades to accommodate factors other than aerodynamic behavior, which is not desirable.